(1) Field of the Invention
The present invention relates to the field of transmission boxes fitted to rotorcraft, serving to transmit motion between at least one driver source and at least one receiver source, such as a member of the rotorcraft that is to be driven. The present invention relates more particularly to arrangements concerning the lubrication of moving members housed in a casing forming part of such transmission boxes.
The present invention relates more specifically to ways of monitoring the operation of such a transmission box relative to any loss of lubricant, usually oil, contained in a casing of the transmission box. Such a loss of lubricant may stem, for example, from a structural sealing failure in the casing and/or from a failure of sealing members of the casing.
(2) Description of Related Art
Rotorcraft are fitted with various transmission boxes for transmitting motion between at least one driver source and at least one receiver source. For example, a rotorcraft power plant for driving at least one rotor is fitted with at least one main gearbox for transmitting power from a driver member of the power plant to the rotor(s). Also by way of example, secondary gearboxes may be engaged with the main gearbox for driving various pieces of equipment and/or accessories of the rotorcraft.
Gearboxes comprise a casing in which various transmission members are installed that are arranged in mutual engagement and that are carried by transmission shafts. The transmission shafts comprise in particular drive shafts and intermediate shafts.
By way of example, drive shafts comprise at least one inlet shaft driven from a driver source and at least one outlet shaft for driving equipment that needs to be moved. One or more intermediate shafts may be interposed between an inlet shaft and an outlet shaft. The transmission shafts engage with one another via transmission members that they carry, and they are installed in the casing via bearing members.
By analogy with gearboxes, bearing boxes are also known for mounting moving members, rotary shafts in particular, that are mounted on bearings in a casing via rolling members, with movement being transmitted from an inlet of at least one shaft to an outlet of at least one shaft, said inlet and said outlet can be arranged on the same shaft or on respective shafts.
The moving members of transmission boxes, such as in particular transmission members, transmission shafts, and/or rolling members, are subjected to heating caused by the friction they suffer. In order to preserve the moving members from such heating, transmission boxes are commonly fitted with a lubrication circuit. A supply of lubricant, in particular of oil, is provided inside the casing. The lubricant flows via a lubrication circuit between the casing and a heat exchanger serving to cool the lubricant.
A general problem that arises lies in the sealing of the casing, which might be faulty. Leaks of lubricant can be observed and it is necessary for the sealing of the casing to be checked regularly.
For example, the casing often comprises a plurality of casing elements that are assembled together. Such assembly constitutes a potential source of leakage through the junction zones where elements of the casing join one another.
Also by way of example, rolling members interposed between the casing and the drive shafts are potential sources of leaks of lubricant. In order to avoid such leaks, the casing and/or the rolling members are commonly provided with sealing members. Nevertheless, wear and/or failure of the sealing members can allow lubricant to escape from the casing.
Also by way of example, casings are commonly provided with at least one vent for allowing air contained inside the casing to escape naturally. Such a natural escape of air is necessary because the air contained in the casing expands under the effect of the temperature rise caused by the moving members heating up. In this respect, reference may be made for example to document DE 10 2007 026338 (MAGNA POWERT AG & CO KG), that proposes preventing oil from escaping through the vent by making use of a deformable membrane that closes the vent by deforming under the effect of the air expanding.
Prior to transmission boxes being put into service, it is common practice to perform operations to verify their sealing. In one conventional method, the transmission box is coated in a substance suitable for revealing zones that are potential sources of lubricant escaping from the casing. The leakage sources that are revealed are subsequently treated individually in order to remedy the observed losses of sealing of the transmission box. The sealing of the transmission box is then verified once more in order to check that the treatment of the sources of leakage is effective. A possible additional verification operation is commonly performed under conditions in which the moving members of the transmission box are put into movement. For this purpose, the casing is filled with a liquid and the moving members are driven for a predetermined duration in order to verify whether any liquid escapes from the casing.
Such techniques for verifying the sealing of transmission boxes are lengthy and expensive, but they are effective and necessary given the safety constraints that apply to aircraft. As a result, it is desirable to avoid as far as possible any occasion on which it is observed that the transmission box is not thoroughly sealed.